This research seeks to examine the specificity of several objective abnormalities on hematologic, serum chemistry, immunologic and virologic testing, that have been described in patients with chronic fatigue syndrome (CFS). In particular, the results of these tests in patients with fibromyalgia and major depression -- two conditions that can present with symptom complexes indistinguishable from CFS -- would be examined. Twenty-five patients with CFS, 25 with fibromyalgia, 25 with depression and 25 age- and sex-matched healthy control subjects would each be tested on two occasions, separated by 6-12 months. The following tests would be performed on coded samples: numbers of leucocytes, monocytes, and atypical lymphocytes; levels of alkaline phosphatase, and total cholesterol; levels of circulating immune complexes, as assessed by a sensitive radiolabeled C1q binding assay; levels of immunoglobulin classes IgG, IgM and IgA; anti- nuclear antibodies; number and percentages of lymphocyte subsets in an attempt to confirm and extend previous research that has indicated these subsets may be abnormal in CFS, as determined by two-color flow cytometric techniques; natural killer (NK) cell function, as assessed by standard chromium-release assay, against various virally-infected or neoplastic target cell line; active replication of human herpesvirus-6 (HHV-6) in peripheral lymphocytes from patients, as determined by quantitative polymerase chain reaction (PCR) and by cell culture techniques, using monoclonal antibodies against HHV-6-specific epitopes. Measures of psychic stress and functional status will be obtained, and used as covariates in multivariate analyses comparing the four groups. If resources permit, we also would study two other patient groups who can present with a symptom complex similar to that of CFS: patients with systemic lupus erythematosus (SLE) and generalized anxiety disorder.